Hot water cools from 60degree celcius to 50 degree celcius in the first 10 min and to 42 degree celcius in the next 10 min .The temp of the surrounding.

average temp. in first case = (60+50) /2 = 55 C

average temp. in second case = (50+42) /2 = 51 C

as frm law of cooling

dT/dt = -k(T-T_o )  where T_o is temp. of surrounding

case 1 : dT = 10 C and dt = 10 mins

so ,  1=-k(55-T_o )         ....(1)

case 2 : dT = 8 C and dt = 10 mins

so ,  0.8=-k(51-T_o )       _.........(2)

From 1 and 2 , we have

Temp of surr. as 35 C

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